Spong, U.S. Pat. No. 4,097,895 issued June 27, 1978 and incorporated herein by reference, has disclosed an optical recording system and an ablative optical recording medium for use therein. The recording medium comprises a light reflective material which is coated with a layer of a light absorptive organic material. A focussed, modulated light beam, such as a light beam from an argon ion laser, when directed at the recording medium vaporizes or ablates the light absorptive layer leaving an opening in this layer and exposing the light reflective material. The reflectivity in the area of the opening in the light absorptive layer is essentially that of the light reflective material and is much greater than that of the surrounding unexposed region. During readout this difference in reflectivity is detected optically and converted into an electrical signal representative of the recorded information.
Credelle et al, RCA Review, 33, 206 (1972), have disclosed the utility of thermoplastic materials for holographic recording. The recording medium comprises a substrate, overlaid sequentially by an electrically conductive coating, a photoconductor and a thermoplastic material. Information is recorded in this medium using the steps of: (1) electrically charging the free surface of the thermoplastic material thus inducing a voltage between the free surface and the conducting layer; (2) exposing the light absorbing photoconductor to a recording light beam thus discharging the voltage across the photoconductor and leaving the surface charge on the thermoplastic unchanged; (3) recharging the free surface thereby producing an increase in surface charge in the exposed areas; (4) heating the thermoplastic material to a softening temperature whereby it deforms in response to the nonuniform electrical field until these forces are balanced by surface tension forces. Alternatively, the electrical charging and exposure are carried out simultaneously or the charging and exposure can be carried out during the heating and/or cooling of the thermoplastic thus reducing the time required for recording. Information so recorded can be erased by heating the uncharged thermoplastic material above its softening temperature.
Gundlach et al, Photographic Science and Engineering, 7, 14 (1963), have disclosed that if the thermoplastic material is heated to its softening temperature while uniformly charged a random deformation of the thermoplastic material in the charged region, known as frost, occurs. This random deformation has a dominant spatial wavelength about twice the thermoplastic layer thickness and is reversible.
Ghekiere, U.S. Pat. No. 4,139,853, issued Feb. 13, 1979, discloses a recording medium consisting of a thermoplastic layer having a light absorbing organic dye dissolved therein to impart light sensitivity to the medium. This patent discloses that, upon exposure to light, minute light scattering centers are formed in the thermoplastic layer. The information recorded therein by this process is not reversible, i.e. it cannot be erased.
Spong's optical recording medium and others similar to it are not reversible since once the light absorbing layer is ablated this layer cannot be returned to its original state without remanufacture of the optical recording medium. A reversible optical storage medium which can be read out immediately after recording is desirable since information stored on the medium could then be updated without rerecording all the information stored therein or alternatively the optical recording medium can be reused after erasure of all the information stored therein.